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Message-ID: <48B55122.6080807@ccur.com>
Date: Wed, 27 Aug 2008 09:05:38 -0400
From: John Blackwood <john.blackwood@...r.com>
To: Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
Mike Galbraith <efault@....de>
Subject: Re: [PATCH] sched_rt_rq_enqueue() resched idle
> On Tue, 2008-08-26 at 15:09 -0400, John Blackwood wrote:
> > > Hi Peter,
> > >
> > > When sysctl_sched_rt_runtime is set to something other than -1
and the
> > > CONFIG_RT_GROUP_SCHED kernel parameter is NOT enabled, we get
into a state
> > > where we see one or more CPUs idling forvever even though there are
> > > real-time
> > > tasks in their rt runqueue that are able to run (no longer
throttled).
> > >
> > > The sequence is:
> > >
> > > - A real-time task is running when the timer sets the rt runqueue
> > > to throttled, and the rt task is resched_task()ed and switched
> > > out, and idle is switched in since there are no non-rt tasks to
> > > run on that cpu.
> > >
> > > - Eventually the do_sched_rt_period_timer() runs and un-throttles
> > > the rt runqueue, but we just exit the timer interrupt and go back
> > > to executing the idle task in the idle loop forever.
> > >
> > > If we change the sched_rt_rq_enqueue() routine to use some of the
code
> > > from the CONFIG_RT_GROUP_SCHED enabled version of this same
routine and
> > > resched_task() the currently executing task (idle in our case) if
it is
> > > a lower priority task than the higher rt task in the now un-throttled
> > > runqueue, the problem is no longer observed.
>
> Very good spotting, Thanks!
You're welcome.
> However I think the patch isn't quite good, as highest_prio is only
> available on SMP || RT_GROUP_SCHED.
>
> Furthermore, on !RT_GROUP_SCHED any RT task will be higher than current,
> so we can do the below, do you agree?
Yes, I see what you are saying.
The patch version below looks good.
I re-tested with it and it works fine.
Thanks!
> diff --git a/kernel/sched_rt.c b/kernel/sched_rt.c
> index 94daace..f672aee 100644
> --- a/kernel/sched_rt.c
> +++ b/kernel/sched_rt.c
> @@ -199,6 +199,8 @@ static inline struct rt_rq *group_rt_rq(struct
sched_rt_entity *rt_se)
>
> static inline void sched_rt_rq_enqueue(struct rt_rq *rt_rq)
> {
> + if (rt_rq->rt_nr_running)
> + resched_task(rq_of_rt_rq(rt_rq)->curr);
> }
>
> static inline void sched_rt_rq_dequeue(struct rt_rq *rt_rq)
>
>
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